Why do Buddhist Stupas have various shapes?

I came across a gallery of Stupas from around the world and it made me wonder why some of them differ in shape considerably. Do they express different aspects of Buddhism or is the source of difference rather cultural/historical? I am aware that in Tibetan Buddhism there are 9 types of Stupas and they have a rather uniform shape. What I am interested in is why the ones found in, for example, Sri Lanka or India, have a much different shape and style.